A belt press normally has a plurality of upper and a plurality of lower traverses respectively having lower and upper traverse faces with the traverses all parallel and each upper traverse lying above a respective lower traverse. Upper and lower heatable press platens are carried between these traverses and in turn have respective upper and lower platen faces respectively confronting the lower and upper traverse faces. Thus a belt to be pressed can be engaged between the platens. Mechanically actuated means including bolts is engaged between each lower traverse and the respective upper traverse for displacing the traverses toward each other and thereby pressing the platens against the belt between them. During a pressing operation the platens bend somewhat from their normally planar shape, as the platens are not made stiff enough to avoid such bending in order to cut down on equipment weight.
Normally some sort of pressing system is provided between the movable platen and the respective traverses. Such a system normally comprises an array of screw-type or hydraulic jacks engaged between this movable platen and the respective traverses, with a row of such jacks between each traverse and the respective movable platen.
When hydraulic jacks are employed, they must be actuated at very high pressure and in a very careful manner so as to bring as even a force as possible to bear on the belt clamped between the press platens. The pressure employed is normally very large, in the neighborhood of several hundred atmospheres. What is more it is necessary to cut out the traverses to accomodate these jacks, substantially weakening the traverses which are in any case made a light as possible to make transport of the machine possible.
It has been suggested to employ pressurizable bladders or cushions as hydraulic actuating means. Such a solution has not worked because no such cushion has been found that can withstand the pressure necessary in a belt press. If pressurized sufficiently such a bladder will frequently blow out or burst laterally.